Holographic gratings formed in the core of optical fibers are finding increasing use in a variety of optical device applications. See A. E. White and S. J. Grubb "Optical Fiber Components and Devices," Ch. 7 in Optical Fiber Telecommunications, Vol. IIIB, T. L. Koch and I. P. Kaminow, ed., Academic Press, pp. 267-318 (New York, 1997). These include fiber lasers, semiconductor laser stabilizers, pump reflectors, dispersion compensators, filters, demultiplexers, and gain equalizers. Such gratings may also be used as strain sensors in architectural applications. The increasing market demand for optical fiber gratings has stimulated the development of manufacturing methods for their mass production.
An optical fiber comprises an inner core of relatively high refractive index, an outer cladding, and a polymer coating. The inner core is made of ultraviolet (UV) photosensitive glass, such as a germanosilicate, so that a grating can be induced by UV irradiation. The dominant method for photo-inducing gratings in optical fibers is by side writing into a stripped fiber with UV light through the fiber cladding. The fiber is exposed to actinic light having an intensity that varies periodically along the length of the fiber. This, in turn, creates periodic refractive index variations in the fiber core. The periodically varying intensity pattern is typically provided by applying a UV beam through an optical phase mask. See Anderson et al, U.S. Pat. No. 5,327,515 issued Jul. 5, 1994. Alternately, the intensity pattern can be provided by an amplitude mask or by interfering a pair of coherent UV beams. See W. H. Glenn et al, U.S. Pat. No. 4,725,110, issued Feb. 16, 1998. In each of these techniques, the source of actinic radiation is typically a high intensity excimer laser operating with an output wavelength near 240 nm. Longer wavelength sources may be used, depending upon the level of photosensitivity of the particular fiber and the degree of index change that is needed.
Frequently, the photosensitivity of the fiber is enhanced for writing by treating it with hydrogen or deuterium under elevated temperature and pressure. The hydrogen or deuterium diffuses through the polymer coating and cladding, and into the core of the optical fiber. Such treatment enhances the photosensitivity of the core so that the grating can be written at a lower intensity or lower dose. The photosensitivity of the core can be further enhanced in a number of known ways, including for example collapsing the preform under a reducing atmosphere in order to increase the concentration of photosensitive sites (U.S. Pat. No. 5,157,747 to R. M. Atkins, K. T. Nelson, K. L. Walker, "Photorefractive Optical Fiber", issued Oct. 20, 1992).
Presently, a time-consuming step in grating fabrication is removing and subsequently reapplying the protective polymer coating that was applied to the fiber at the time of drawing. The coating is needed to protect the sensitive fiber from contamination and mechanical damage, but typical coatings are highly UV absorbent and inhibit grating formation. This is, in part, because most commercial fiber coatings are UV curable and incorporate highly absorbing photoinitiators in order to initiate the curing process. Such coatings are damaged by UV laser beams. Upon exposure to UV, the coatings block the light via absorption, converting the energy to heat. They discolor and will eventually ablate, given sufficient exposure. Thus, an initial step in conventional grating writing is stripping the polymer coating, as by soaking the fiber in hot sulfuric acid. The coating removal and reapplication steps may consume more than half the time required to fabricate a grating.
In an alternate fabrication technology, gratings have been written into the fiber during drawing before application of the coating. See L. Dong, J.-L. Archambault, L. Reekie, P. St. J. Russell, D. N. Payne "Single Pulse Bragg Gratings Written During Fibre Drawing," Electron.Lett., 29(17), 1577 (1993). A limitation of these gratings is that they typically have low reflectivities due to the single pulse exposure. An earlier patent, U.S. Pat. No. 5,620,495, incorporated herein by reference, demonstrated that, with the proper selection of low absorbing polymer, gratings could be formed by exposing the core through the polymer coating and cladding to a pattern of UV radiation. Following this teaching, the coating must be substantially transparent to the UV radiation used to form the grating. Typical polymer coating materials used in commercial practice have minimal UV transparency, particularly near 240 nm, owing largely to the presence of UV absorbing photoinitiators. The patent referenced above teaches various polymer coating materials that can be formulated to be sufficiently UV transparent to allow the formation of gratings using radiation directed through the coating, e.g., alkyl-substituted silicones and silsesquioxanes, aliphatic polyacrylates, polymethacrylates and vinyl ethers. The cited examples utilized a thermally cured methylsilsesquioxane coating which, as such, did not contain UV absorbing photoinitiators and provided good UV transparency. Claesson et al subsequently demonstrated a fiber grating written through a thin solvent-deposited coating of Teflon AF, a fluoropolymer that likewise did not contain UV photoinitiators. See A. Claesson, B. Sahlgren, M. Fokine, R. Stubbe, "UV-transparent Coatings for Optical Fiber," Proc. of 46.sup.th Int. Wire & Cable Sym., pp.82-85 (1997).
Thus, both thermal curing and solvent deposition processes have been used to provide UV transparent coatings by avoiding the use of UV absorbing photoinitiators. However, a UV curable coating is preferred in order to be compatible with conventional fiber draw processes. The benefits of UV curable coatings include a more rapid rate of hardening, the capability of applying relatively thick coatings with minimal shear stress on the fiber, superior control of viscosity during the coating process, avoidance of solvent usage, and the elimination of high-temperature curing or evaporating furnaces. However, optical fiber coatings that are UV curable while also maintaining good UV transparency in the mid-UV range near 240 nm are difficult to produce using conventional technologies.
In a more recent patent, U.S. Pat. No. 5,773,486, UV curable coatings are disclosed that have substantial UV transparency in the range of 240-260 nm. This is achieved using novel low UV absorbing free radical photoinitiators. However, the disclosed coating resins are based on the use of free radical chemistry and the cited examples use acrylate monomers, wherein the attainable UV transparency is limited due to the presence of C.dbd.O functional groups which are weak, but significant, UV absorbers.
Conventional optical fiber coatings often have significant transparency in the near-UV range, i.e., at wavelengths greater than .about.300 nm, and some researchers have attempted to exploit that window for writing through the coating. For example, Starodubov et al reported a .about.1 dB grating written through a conventional fiber coating using 334 nm light and later, a .about.15 dB grating written through a 40 .mu.m silicone coating at 302 nm. (See D. S. Starodubov, V. Grubsky, J. Feinberg "Efficient Bragg Grating Fabrication in a Fibre Through Its Polymer Jacket Using Near-UV Light," Electron. Lett., 33, pp. 1331-1332 (1997) and D. S. Starodubov, V. Grubsky, J. Feinberg, E. M. Dianov, S. L. Semjonov, A. N. Guryanov, N. N. Vechkanov "Fiber Bragg Gratings with Reflectivity&gt;97% Fabricated Through Polymer Jacket Using Near-UV Light," Bragg Gratings, Photosensitivity, and Poling in Glass Fiber and Waveguides: Applications and Fundamentals Topical Meeting, Optical Society of America, Williamsburg, Va., post-deadline paper PD1-1, Oct. 26-28, 1997. Because the optical absorption in the fiber core at 334 nm and 302 nm is much weaker than at 244 nm, ( See R. M. Atkins "Measurement of the Ultraviolet Absorption Spectrum of Optical Fibers," Opt. Lett., 17 pp. 469-471 (1992)), gratings written at longer wavelengths require unusually high UV intensities, as much as 1 kW/cm.sup.2, very high germanium concentration (.about.20 mol %) and in some cases the addition of boron as a co-dopant to increase the photosensitivity of the fiber. Such high levels of germanium make it difficult to achieve low-loss splices to standard communication grade fibers, and are not highly desirable.
A number of previous practitioners have cited the utility of vinyl ethers for formulating optical fiber coatings. See e.g. S. A. Shama, E. S. Poklacki, J. M. Zimmerman "Ultraviolet-curable cationic vinyl ether polyurethane coating compositions" U.S. Pat. No. 4,956,198 (1990); S. C. Lapin, A. C. Levy "Vinyl ether based optical fiber coatings" U.S. Pat. No. 5,139,872 (1992); P. J. Shustack "Ultraviolet radiation-curable coatings for optical fibers" U.S. Pat. No. 5,352,712 (1994); J. R. Petisce "Optical fiber including acidic coating system," U.S. Pat. No. 5,181,269 (1993). However, prior art vinyl ether-based optical fiber coatings have not been formulated expressly to achieve high UV transparency, especially at wavelengths as low as 240 nm. Neither, apparently, have vinyl ethers been used to formulate coatings for other applications requiring high UV transparency at wavelengths as low as 240 nm.
Various oligomers or polymers that were not vinyl ether functionalized have been used in coating formulations with vinyl ether monomers. Such oligomers/polymers have included non-reactive resin fillers such as cellulose derivatives (See S. C. Lapin "Semi-interpenetrating polymer networks" U.S. Pat. No. 4,654,379), as well as reactive acrylate-functional oligomers, unsaturated polyester oligomers (See C. E. Bayha "Cationically initiated curable resin system" U.S. Pat. No. 5,252,682), or epoxy-functional oligomers (See J. A. Dougherty, F. J. Vara, and L. R. Anderson "Vinyl Ethers for Cationic UV Curing," Radcure'86 Conf. Proc., 15-1, Soc. Manuf. Eng., Dearborn, Mich. (1986), and J. A. Dougherty, F. J. Vara, and L. R. Anderson "Vinyl Ethers for Cationic UV Curing," Radcure'86 Conf. Proc., 15-1, Soc. Manuf. Eng., Dearborn, Mich. (1986). While certain of these components might conceptually provide good UV transparency, apparently no attempt has been made to incorporate them in vinyl ether coatings so as to optimize UV transparency, especially in the wavelength region of 240-300 nm. For example, U.S. Pat. No. 4,654,379 only cites examples which use a high level (4 phr) of an aromatic onium salt catalyst, which would have poor UV transparency. The epoxy-functional oligomers that are commercially available (e.g., the diglycidyl ether of Bisphenol A or its derivatives) often contain aromatic groups that are unacceptable for UV transparency and also increase the tendency toward yellowing. In general, the commercially available cycloaliphatic (non-aromatic) epoxies do not have sufficiently high molecular weights of themselves to provide adequate viscosity for optical fiber coating applications.